Drupal 8: validate entity field if value changed (PHP 7).

MODULE.module

<?php

/**
 * Implements hook_entity_type_alter().
 */
function MODULE_entity_type_alter(array &$entity_types) {
  /* @var \Drupal\Core\Entity\ContentEntityType $entity_user */
  $entity_user = $entity_types['user'];
  $entity_user->setFormClass('default', \Drupal\MODULE\Entity\User\Form\ProfileForm::class);
}

ProfileForm.php

<?php

namespace Drupal\stats\Entity\User\Form;

use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Field\FieldItemListInterface;
use Drupal\user\Entity\User;
use Drupal\user\ProfileForm as BaseProfileForm;

/**
 * @method User getEntity()
 */
  class ProfileForm extends BaseProfileForm {

  /**
   * Validate entity field.
   *
   * @param array $form
   *   Form specification.
   * @param FormStateInterface $form_state
   *   A state of form.
   * @param string $field
   *   Name of field.
   * @param \Closure $validator
   *   Validation callback.
   */
  protected function validateEntityField(array &$form, FormStateInterface $form_state, string $field, \Closure $validator) {
    $entity = $this->getEntity();

    if ($entity->hasField($field)) {
      try {
        $value = $form_state->getValue($field);

        // Execute validation only if value have been changed.
        if ($entity->get($field)->getValue() !== $value) {
          // Update the value of entity field.
          $entity->set($field, $value);
          // Validate updated value.
          $value = $validator->call($this, $entity->get($field));

          // Update the entity and state of form if value has been processed.
          if (NULL !== $value) {
            $entity->set($field, $value);
            // Update state of form with new, validated value.
            $form_state->setValue($field, $entity->get($field)->getValue());
          }
        }
      }
      catch (\Exception $e) {
        $form_state->setError($form[$field], $e->getMessage());
      }
    }
  }

  /**
   * {@inheritdoc}
   */
  public function validateForm(array &$form, FormStateInterface $form_state) {
    $this->validateEntityField($form, $form_state, 'field_first_name', function (FieldItemListInterface $field) {
      // Remember that object instance is accessible inside of this
      // anonymous callback.
      switch ($field->value) {
        // Reject entity saving.
        case 'Dummy':
          throw new \InvalidArgumentException($this->t('Incorrect name!'));

        // Convert the value.
        case 'Sergey':
          return 'Sergii';

        // Allow value as it is.
        default:
          return NULL;
      }
    });

    return parent::validateForm($form, $form_state);
  }

}